Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.

All information regarding Constant Contact's API can be found within The Constant Contact Developer Portal. Constant Contact provides all developers with free access to our V3 API endpoints for building their own application for integration. All active account types, including trial , have free access to Constant Contact's public API. 

You can get started using our API by visiting our documentation here:

https://v3.developer.constantcontact.com/api_guide/getting_started.html

While we don't have dedicated developer/sandbox accounts per se, you are able to create a free developer trial account with limited sending capabilities which you can use for your testing purposes. Trial accounts created through the developer portal do not expire, whereas standard trial accounts created through the main site expire after 60 days if not converted to a paid account. Below is a link to our developer portal where you can sign up for a trial account, you'll just need to click on "Sign up".

Constant Contact Developer Portal

https://v3.developer.constantcontact.com/login/index.html

As with most APIs, our API has rate limits to help maintain stability and security. Our standard API key access has a rate limit of 10,000 calls per day, and 4 calls per second. Once a key has hit its limit, your calls will return a 429 error until the next day (or the next second if going over your queries-per-second limit).

429: Too Many Requests

https://v3.developer.constantcontact.com/api_guide/glossary_responses.html#429-too-many-request  

If you are planning on making an application that would be available and potentially used by a large base of customers then I would suggest you look into becoming a Technology Partner with Constant Contact. There is no price involved with this (though your application must be approved) and the basics are to just create and maintain your app, though there are other parts to it. Members of our partner program are given 250,000 calls per day and up to 10 calls per second. To associate your API Key with a partnership, you can look into becoming a Technology Partner: https://www.constantcontact.com/partners/technology  

I am uncertain as to the turnaround times regarding Technology Partner applications, but that team does offer phone support, so if desired, you could definitely reach out to our Partner Support line at 1-855-854-8609 and see if they are able to provide any additional details or timelines on that process.

Please have a look and let us know if you have any other questions!

That's excellent. I think that covers everything we need to know. Thank you for the quick response here!

In terms of my comment about the pricing page, I definitely have all the information I need based on your reply, but I think an addition to that page would be helpful for those investigating your platform.

Maybe something in the "Additional Features" section ("Access to Constant Contact's API"), and the daily limits in each column ("10,000 calls per day", "10,000 calls per day", "10,000 calls per day") so that people know what the daily cap is.

Thanks for the quick response here. Very much appreciated.